/Engine/Source/Runtime/SlateCore/Public/Fonts/FontCache.h
The indices used here are relative to the start of the text we were shaped from, even if we were only shaped from a sub-section of that text
The measured width, or an unset value if the text couldn't be measured (eg, because you started or ended on a merged ligature, or because the range is out-of-bounds)
[TOptional](API\Runtime\Core\IO\TOptional)< int32 > GetMeasuredWidth ( const int32 InStartIndex, const int32 InEndIndex, const bool InIncludeKerningWithPrecedingGlyph ) const